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Table 2.6 Conversion factors for units of pressure



    Kpa   Bar   Std Atm   kg.f/cm2   Ib.f/inch2 (p.s.i.)   Ib.ft/ft2 (p.s.i.)   mm (Mercury)   inches (Mercury)   inches (Water)   (Water) feet   metres (Water)  
Kpa   1   0.01   0.0099   0.0102   0.1450   20.88   7.50   0.2953   4.015   0.3346   0.1020  
Bar   100   1   0.9869   1.020   14.50   2,089   750.1   29.53   402.2   33.52   10.22  
Std Atm   101.325   1.013   1   1.033   14.70   2,116   760   29.92   407.5   33.96   10.35  
kg.f/cm2   98.039   0.9807   0.9678   1   14.22   2,048   735.6   28.96   394.4   32.87   10.02  
Ib.f/inch2 (p.s.i.)   6.8966   0.06895   0.06805   0.07031   1   144   51.72   2.036   27.73   2.311   0.7044  
Ib.f/ft2   0.0479   4.788x10-4   4.725x10-4   4.882x10-4   0.006944   1   0.3591   0.01414   0.1926   0.01605   0.004891  
mm Hg   0.1333   0.001330   0.001316   0.001360   0.01934   2.785   1   0.03937   0.5362   0.04469   0.01362  
inches Hg   3.3864   0.03386   0.03342   0.03453   0.4912   70.73   25.4   1   13.62   1.135   0.3459  
inches H2O   0.2491   0.002486   0.002454   0.002535   0.03606   5.193   1.865   0.07342   1   0.0833   0.02540  
ft H2O   2.9886   0.02984   0.02944   0.03042   0.4327   62.31   22.38   0.8810   12   1   0.3048  
metres H2O   9.8039   0.09789   0.09660   0.0998   1.420   204.4   73.42   2.891   39.37   3.281   1  

 

Note: 1 Kpa (kilopascal) = 1 kilonewton/m2             Std Atm = standard atmospheres


temperatures. Decreasing the pressure above the liquid lowers the boiling point and increasing the pressure raises the boiling point. The curve marked 'P' in Figure 2.12 illustrates the variation in saturated vapour pressure with temperature for propane. It will be noticed that an increase in liquid temperature causes a non-linear increase in the saturated vapour pressure. The non-linear shape of the curve shows also that the saturated gas does not behave exactly in accordance with the Gas Laws (see also Figure 2.9(c)). Also shown on Figure 2.12 are the variations of propane liquid density (y') and saturated vapour density (y") with temperature.

Different liquefied gases exert different vapour pressures. This can be seen from Figures 2.13 and 2.14. The vertical axis in these two figures gives the saturated vapour pressure on a logarithmic scale. (The use of the logarithmic scale changes the shape of the curves from that shown for 'P' in Figures 2.7 and 2.12). Figure 2.13 shows information for the hydrocarbon gases. A comparison of the graphs shows that smaller molecules exert greater vapour pressures than larger ones. In general the chemical gases shown in Figure 2.14 exert much lower saturated vapour pressures than the small hydrocarbon molecules such as methane. The point of intersection of these curves with the horizontal axis indicates the atmospheric boiling point of the liquid (the temperature at which the saturated vapour pressure is equal to atmospheric pressure). This is the temperature at which these cargoes would be transported in fully refrigerated or fully insulated containment systems.

Whereas the bar is now the most frequently used pressure unit in the gas industry, other units such as kgf/cm2 (kilogrammes force per square centimetre), atmoshpheres or millimetres of mercury are frequently encountered. However, the only legal units are the Sl units with kilopascal as the usual pressure unit. The conversion factors for these units of pressure are given in Table 2.6.

All gauges used for the measurement of pressure measure pressure difference. Gauge pressure is therefore the pressure difference between the pressure to which the gauge is connected and the pressure surrounding the gauge. The absolute pressure is obtained by adding the external pressure (such as atmospheric pressure) to the gauge pressure.

Vapour pressures, though they may be found by means of a pressure gauge, are a fundamental characteristic of the product. Accordingly, they are essentially absolute pressures. Tank design pressures and relief valve settings, however, like pressure gauge indications, are tuned to the physical difference between internal and external pressure and thus are gauge pressures. For consistency throughout this book, most pressures are given in bars but, to avoid confusion, the unit is denoted as barg where a gauge pressure is intended.

It is appropriate to re-emphasize here the information from Chapter One that a liquefied gas is defined in terms of its vapour pressure as a substance having a vapour pressure at 37.8°C equal to or greater than 2.8 bar absolute.

2.16 LIQUID AND VAPOUR DENSITIES

2.16.1 Liquid density

The density of a liquid is defined as its mass per unit volume and is commonly measured in kilogrammes per cubic decimetre (kg/dm3). Alternatively, liquid density may be quoted in kg/litre or in kg/m3. The variation with temperature of the liquid density of a liquefied gas (in equilibrium with its vapour) is shown for propane in curve y' in Figure 2.12. As can be seen, the liquid density decreases with increasing temperature. The large changes seen are due to the comparatively large coefficient of


volumetric expansion of liquefied gases. Values for liquid density (relative to water) of liquefied gases at their atmospheric boiling points are quoted in Table 2.5. All the liquefied gases, with the exception of chlorine, have liquid relative densities lower than one. This means that in the event of a spillage onto water, these liquids would float prior to evaporation.



Rollover

A danger associated with cargo density is the phenomenon of rollover. The conditions for rollover are set when a tank's liquid contents stratify so that a heavier layer forms above a less-dense lower layer (see Reference 2.35). Rollover is the spontaneous mixing which takes place to reverse this instability. Rollover, in either a ship or shore tank, can result in boil-off rates ten times greater than normal, causing over-pressurisation, the lifting of relief valves and the release to atmosphere of considerable quantities of vapours or even two-phase mixtures.

When liquids of differing density are loaded — without mixing — into the same tank, there is a possibility that layering will take place. For LNG tanks, this may be as a result of the older contents becoming weathered between cargo imports because of genuine density differences or by a high concentration of nitrogen in the feed gas. In the case of LPGs, it may be due to cargo mixing (see below). Instability will occur between the layers if the lower layer becomes less dense than the upper. This can happen due to heat input to the lower portion while (in the case of LNG) evaporation of methane is taking place in the upper layer, leaving a higher percentage of the heavier ends {weathering).

The phenomenon is largely limited to LNG storage tanks, although it is known to have occurred on LNG carriers. Furthermore, a number of recorded rollover incidents involving the shore storage of ammonia are known. For most other liquefied gases, being pure products, the risk of rollover is less severe as the process of weathering will be limited. However, if two different cargoes, such as butane and propane, are loaded into the same tank, layering can become acute. Loading a ship's tank by this means is not recommended unless a thorough thermodynamic analysis of the process is carried out and the loading takes place under strictly controlled conditions.

The following are measures which can help prevent rollover:

• Store liquids of differing density in different shore tanks

• Load shore tanks through nozzles or jets to promote mixing

• Use filling pipework at an appropriate level in the shore tank

• Do not allow prolonged stoppages when loading ships

• Monitor cargo conditions and boil-off rates for unusual data

• Transfer cargo to other tanks or recirculate within the affected shore tank

2.16.2 Vapour density

The density/temperature relationship of the saturated vapour of propane is given by curve y' in Figure 2.12. The density of vapour is commonly quoted in units of kilogrammes per cubic metre (kg/m3). The density of the saturated vapour increases with increasing temperature. This is because the vapour is in contact with its liquid and, as the temperature rises, more liquid transfers into the vapour-phase in order to achieve the higher vapour pressure. This results in a considerable increase in mass per unit volume of the vapour space. The densities of various vapours (relative to air) at standard temperature and pressure are given in Table 2.5. Most of the liquefied gases


produce vapours which are heavier than air. The exceptions are methane (at temperatures greater than -113°C), ethylene and ammonia (see also 10.1.2). Vapours released to the atmosphere, which are denser than air, tend to seek lower ground and do not disperse readily.

2.17 PHYSICAL PROPERTIES OF GAS MIXTURES

If the components of a gas mixture are known, it is possible to perform a variety of calculations using the following relationships.


Molecular mass

Molecular mass of gas mixture = MiVi/100 where Mi = component molecular mass

Vi = percentage component volume

Percentage mass

Percentage mass of component = ViMi/Mmix

where Mmix = molecular mass of gas mixture

Relative vapour density

Relative vapour density of gas mixture (at 0°C and 1 bar) = Mmix/Ma

where Ma = molecular mass of air = 29

For example, given the percentage by volume of the components in a gas mixture, Table 2.7 shows how the molecular mass of the mixture can be determined. The example taken considers the composition of a typical natural gas.


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